1. Field of the Invention
The present invention relates to counters for counting particles suspended in a fluid medium and more particularly to counters using pulse width integration for measuring paticles per unit volume of fluid medium.
2. Description of the Prior Art
The prior art in the broad field of particle counting may be described by considering the more limited field of blood cell counting. Heretofore, blood cell counting was accomplished in three distinctly different ways. Firstly, a sample of diluted blood was placed under a microscope and the blood cells were individually counted to determine the blood cell count. Secondly, a more sophisticated blood cell counting system utilized optics and photoelectric cells to provide a count of the number of blood cells passing through a light beam. Thirdly, pure electronic blood cell counters were utilized wherein changes in conductivity of a diluted blood sample were sensed to provide an output corresponding to the number of blood cells in the sample.
Some of the less sophisticated pure electronic blood cell counters merely applied a signal across a volume of diluted blood and utilized the voltage developed across the blood sample as an indication of the number of blood cells present. This type of device is inaccurate and did not prove satisfactory.
A more sophisticated type of pure electronic blood cell counter was developed wherein a known volume of diluted blood was drawn through an aperture and a DC electric signal was applied to electrodes positioned at each side of the aperture to develop a voltage corresponding to the instantaneous conductivity of the blood sample passing through the aperture. Since blood cells have extremely low conductivity as compared to the diluent in which they are diluted, each time a blood cell passed between the electrodes, the voltage between the electrodes would increase and provide a pulse output. When the pulses exceeded a predetermined threshold level, a blood cell was counted.
The DC type devices suffered from unique problems relating to electrode stability. Base line instability occurred because of bubble formation and the devices were sensitive to polarization potentials.
To provide a more accurate blood cell count, some of the devices of the prior art volumetrically controlled the amount of blood sample passed through the aperture and initiated and stopped counting based on volume measurements. These devices were expensive and subject to errors because of the difficulty in making accurate volume measurements.
Another problem encountered by the aperture type systems of the prior art was the correction for blood cell coincidence. The devices of the prior art generated a single pulse output for every detected blood cell passing through the aperture. However, when blood cells passed through the aperture in total or partial coincidence, only a single pulse would result and one cell would be counted. In order to correct for this error, the devices of the prior art provided correction tables so that a count of the total output pulses could be converted to a more accurate count by the addition of a statistically determined number of cells. As can be seen, such a system is subject to errors and cannot be said to provide an actual count of the blood cells passing through the aperture.
Another problem experienced with aperture type systems was the accumulation of lint and dust in the aperture, which caused the aperture to become clogged resulting in errors in the blood cell count. In order to detect aperture clogging, the devices of the prior art provided oscilloscopes to view a trace of the voltage generated across the aperture so that any abnormality could quickly be detected and a viewing screen was provided so that aperture could be continuously observed to determine if clogging was taking place.
The formation of bubbles around the aperture caused faulty blood cell counts in the prior art devices. Each time a bubble would pass through the aperture, an additional blood cell was counted thereby resulting in an inaccurate blood count.
Thus, the devices of the prior art required the use of correction tables and did not provide direct readings of the actual blood cell count. In addition, these devices were large and expensive, thereby making them impractical for use by individual doctors, veterinarians and small laboratories.